1. Field of the Invention
The present invention relates to a control apparatus applied to a speed control or a position control in an industrial robot, a numerical control (NC) machine, a head track seeking of a disk drive or the like.
2. Description of the Related Art
In general, a servo mechanism using a servo actuator has been widely utilized in a position-to-position high speed positioning control or a high speed tracking control for an industrial robot, a NC machine and the like, and a head track seeking or the like for a hard disk drive or a floppy disk drive used in a data processing unit. In the servo mechanism, a high speed response is one of very important conditions for such a control as a speed control, a positioning control or the like.
Now, in a control apparatus which controls an object including a servo mechanism having various transfer functions of secondary delay, it has been known that the settling time becomes a minimum time, when a critical damping wherein a damping coefficient of a control system constructing a control apparatus becomes 1 is made under a condition that an overshoot does not occur. That is, in a conventional control system constructed by an inner loop for a negative feedback of an output and another inner loop for negative-feeding back the product of the differential value of the output and a gain, the settling time may be set to be minimum by setting the gain in the another inner value in a proper value. Namely, the response of the control system of the conventional control apparatus is expressed by the following differential equation (1).(d2x/dt2)+{(J+k1)/JT}·(dx/dt)+(1/JT)·x=(1/JT)r  (1)where J is a constant, T is a time constant, and k1 is a gain.
Therefore, the gain k1 can be set such that the damping coefficient ξ=J+k1/2√{square root over (JT)}=1 is obtained. However, when the damping coefficient ξ exceeds 1, the response becomes slower, so that the settling time is prolonged. Also, when the damping coefficient ξ becomes less than 1, the response becomes faster but vibrating even when an overshoot is allowed, so that much improvement can not be obtained.
Accordingly, in a field that the settling time is set to be shorter than that at the time of the critical damping and the high speed response is required, a new control approach is required. In a case that such a control system is constituted on the basis of the following equation (2),(d2e/dt2)+{(2ξWn)/(1+|e|)}·(de/dt)+Wn2e=0  (2)where Wn is natural frequency and e is deviation, and it is considered that, when the deviation between the input value and the output value is large, a control is conducted so as to approach to a steady state by reducing an apparent damping coefficient ξ′=ξ/(1+|e|) to make the response fast but, when the deviation e becomes small, a control is conducted so as to suppress an overshoot by increasing the apparent damping coefficient. However, according to this approach, there is a problem that, since the response is expressed by a nonlinear differential equation, when the deviation becomes large even in a case of ξ=1 in the equation (2), an overshoot occurs.
An object of the present invention is to provide a control apparatus which reduces the settling time largely without occurrence of an overshoot and which achieves a high speed response.